Polyolefins have been made chiefly using conventional Ziegler catalyst systems, but in recent years, the replacement of Ziegler catalysts by metallocene-based systems has begun. Metallocene catalysts, which are transition metal compounds bearing one or more cyclopentadienyl (“Cp”) ring ligand(s), are typically used with aluminoxanes as activators to give very high activities. In many cases, the transition metal is titanium or zirconium. Metallocene polyolefin catalysts provide solutions to many of the problems encountered for Ziegler catalysts (such as low activity, staining and instability from residual catalysts, broad molecular distribution, and ineffective co-monomer incorporation) and are well known in the art.
The commercialization of metallocene catalysts for olefin polymerization has resulted in great interest in the design of non-metallocene homogeneous catalysts. A new generation of catalysts may display improved activity and offer a superior route to known polyolefins and may also lead to processes and products that are outside the capability of metallocene catalysts. In addition, substituted analogues of non-cyclopentadienyl ligands and compounds may be easier to synthesize and hence non-metallocene catalysts may be more cost-effective.
Non-metallocene polyolefin catalysts with at least one phenolate group are well known in the art (see U.S. Pat. No. 4,452,914 to Coleman, III, et al. and U.S. Pat. No. 5,079,205 to Canich). U.S. Pat. No. 5,840,646 to Katayama et al. and EP 0 606 125 B1 assigned to Shell International Research disclose bidentate bis(phenolate) titanium and zirconium catalysts for olefin polymerization.
Multidentate anionic oxygen- and nitrogen-based groups have attracted attention as ligands for non-metallocene polyolefin catalysts. In terms of bidentate ligands, pyridinoxy and quinolinoxy ligands have been reported (see, e.g., U.S. Pat. No. 5,637,660 to Nagy et al.; U.S. Pat. No. 5,852,146 to Reichle et al.; U.S. Pat. No. 6,020,493 to Liu; Bei et al., Organometallics 17:3282 (1997); and Tsukahara et al., Organometallics 16:3303 (1997).)
Tetradentate anionic ligands containing amine-bis(phenolate) groups (phenolate being an aromatic hydroxyl group) have recently been applied in polyolefin catalysts by Kol, Goldschmidt and coworkers (see U.S. Pat. No. 6,333,423 to Kol et al.; Tshuva et al., Chem. Commun. 379 (2000) and Chem. Commun. 2120 (2001)). Shao et al., Organometallics 19:509 (2000) describe zirconium complexes of tridentate chelating amine-bis(alkoxide) (alkoxide being an aliphatic hydroxyl group) ligands as polyolefin catalysts, but the observed activity is very low. Bouwkamp et al., Organometallics 17:3645 (1998) described zirconium complexes with symmetric tridentate amine-bis(σ-aryl) dianionic ligands as polyolefin catalysts, but the observed activities are only moderate.
Hence there is a need in the art for new olefin polymerization catalysts, particularly catalysts containing multidentate ligands of the pyridine-phenolate type. There is also a need in the art for the discovery and optimization of non-metallocene polyolefin catalysts containing unsymmetric or chiral ligands, because this may result in the stereoselective polymerization of 1-olefins (alpha-olefins) and lead to polyolefins with distinctive morphology and properties.
Metallocene catalysts, especially those that are chiral and/or of low symmetry, are used to produce stereoregular polyolefins (see, e.g., G. W. Coates, Chem. Rev. 100:1223 (2000), and references cited therein). These catalysts depend on simple steric effects to control stereoselectivity.
Use of weak non-covalent attractive interactions to achieve stereoselectivity has not been established in the art. Use of weak non-covalent attractive interactions to stabilize reactive intermediates in olefin polymerization has not been established in the art.